Titin Develops Restoring Force in Rat Cardiac Myocytes

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Abstract
When relaxed after contraction, isolated cardiac myocytes quickly relengthen back to their slack length. The molecular basis of the force that underlies passive relengthening, known as restoring force, is not well understood. In a previous study of titin's elasticity in cardiac myocytes, we proposed that titin/connectin develops restoring force, in addition to passive force. This study tested whether titin indeed contributes to the restoring force in cardiac myocytes. Skinned rat cardiac myocytes in suspension were shortened by ≈20%, using Ca2+-independent shortening, followed by relaxation. Cells were observed to relengthen until they reached their original slack sarcomere length. However, the ability to relengthen was abolished after cells had been treated for 12 minutes with trypsin (0.25 μg/mL, 20°C). Gel electrophoresis showed that this treatment had degraded titin without clearly affecting other proteins, and immunoelectron microscopy revealed that the elastic segment of titin in the I band was missing from the sarcomere. Restoring force was also directly measured, before and after trypsin treatment. Restoring force of control cells was −61±20 μg (per cell) at a sarcomere length of 1.70 μm. Comparison of our results with those of activated trabeculae indicated that a large fraction of restoring force of cardiac muscle originates from within the myocyte. Restoring force of myocytes was found to be depressed after titin had been degraded with trypsin. We conclude that cardiac titin indeed develops restoring force in shortened cardiac myocytes, in addition to passive force in stretched cells, and that titin functions as a bidirectional spring. Our work suggests that at the level of the whole heart, part of the actomyosin-based active force that is developed during systole is harnessed by titin, allowing for elastic diastolic recoil and aiding in ventricular filling.